How do trees transport water from their roots to their leaves

A. There are several mechanisms going on here in order for trees, even enormous redwoods that can grow up to 100 metres, to get water from the roots all the way to the top, known as the crown.

Q. What does the water travel in

A. Water and mineral nutrients, known as sap flow, move from the roots to the top of the tree through thin, hollow tubes in the wood tissue, or xylem.

Deciduous tress have very different tubes, called vessel elements, from coniferous trees, whose tubes are called tracheids.

Q. What propels water through the tubes

A. Right from the beginning, when the tree is a mere seedling, a continuous column of water is formed. Two forces maintain the column: one that pulls and one that pushes water up through the tree.

Q. What does the pushing

A. Water is pushed along courtesy of two actions: capillary action and root pressure.

Capillary action, which actually only accounts for a small amount of the pushing force, happens because water is a unique liquid with some special characteristics. Water is really good at sticking to anything it comes into contact with, additionally water is also very good at sticking to itself, or rather its molecules are. As a result, water tends to rise up thin tubes.

Q. If capillary action only does a small amount of pushing, what does the rest

A. Root pressure, created by the movement of water in the root tissue, does most of the pushing power.

Q. And root pressure can push the water up to the highest trees

A. No, it can only push the water up about 2 or 3 metres. To make it to the top of most trees, some of which can be 100 metres high, some pull is needed.

The leaves, or rather the loss of water through them, or evapotranspiration to give it its official name, supplies the pull.

As water evaporates out of the leaves, more water is sucked up the tubes to replace it, maintaining a continuous pull.

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by Lisa Cardy